Developments in phase-comparison techniques for distance protection

Phase-comparison methods of transmission-line protection by distance-to-fault measurement have been widely used in schemes based on electromechanical relaying elements, but investigations into logical methods of protection have exposed an incompleteness in its steady-state theoretical basis, and an only partial utilisation of the techniques that are available in a field which is appreciably wider than it appears is generally supposed. Only the simplest modes of comparison appear to have been sufficiently attractive to warrant application hitherto, and the purpose of the paper is therefore to provide a thorough account of a comprehensive theoretical basis for comparison and to demonstrate the improved steady-state polar characteristics than can be derived from its use. Comparator models which effect phase comparison of two or more relaying signals are described in their schematic form, and composite mho/reactance and mho/ohm characteristics which can be obtained from them are developed. A quadrilateral characteristic, of potentially general application, is described.